Hello, I have just bought a late '60s Fiat 1100t (217) van, as it has rails for the chassis I will do just as you said, make a transporter from it. There is one here in Perth, Australia used for Racing many years ago by Cecchele Motors (Frank), he no longer races his Abarth 131's or Uno turbo or the Lancia Deltas 1 ...but keeps and uses his much loved 241 he converted to a transporter, he won't sell it I have tried! so I build my own. here is a picture of Frank's transporter and my van as purchased.
Regards,Rubino